The invention relates to a color picture screen, in particular for a color cathode ray tube or a color monitor, provided with a picture screen glass, a phosphor layer which is provided on the inner surface of the picture screen glass and comprises red, green, and blue phosphors, and with a color filter layer between the picture screen glass and the phosphor layer.
Color picture screens and color monitors are often used in bright ambient light. Their picture screens should be free from glare, low-reflection, and rich in contrast so as to be better visible under these light conditions and to be less tiring on the eyes.
It is important for achieving a sufficient picture contrast in daylight that the color picture screen should have a highest possible luminance accompanied by a lowest possible reflection of external light. The characteristic quantity defining this property is the so-called Luminance Contrast Performance (LCP):   LCP  =                    Luminance            ⁢              xe2x80x83            ⁢              (        L        )                            Reflection        ⁢                  xe2x80x83                ⁢                  (          R          )                    
An increase in the contrast and accordingly an improvement of the LCP value may be achieved, for example, by means of color filters in the form of inorganic pigments, which are chosen such that they are as transparent as possible to the color emitted by the respective phosphor while absorbing the remaining spectral components. These color filters are provided as separate layers between the phosphor layer and the picture screen. Thus the green and blue components of the incident ambient light are absorbed by a red pigment, the blue and red components by a green pigment, and the green and red components by a blue pigment. In addition, these transparent color filters improve the color purity of the light emitted by the phosphor.
The green pigments used as green color filters contribute very little to the improvement of the LCP value in many cases. U.S. Pat. No. 5,942,848 describes a color picture screen which comprises no color filter between the green phosphor and the picture screen.
It is an object of the present invention to provide a color picture screen which supplies a picture which is rich in contrast.
The object is achieved by means of a color picture screen provided with a picture screen glass, a phosphor layer which is provided on the inner surface of the picture screen glass and comprises red, green, and blue phosphors, and with a color filter layer between the picture screen glass and the phosphor layer, which color filter layer comprises a red color filter layer in the region of the red phosphors in the phosphor layer, a blue color filter layer in the region of the blue phosphors in the phosphor layer, and a blue or a red color filter layer in the region of the green phosphors.
Surprisingly, the red or blue color filter layer in the region of the green-emitting phosphors has advantageous effects. Thus the LCP value of the entire color picture screen is improved. With a red color filter layer between the picture screen glass and the green phosphor, furthermore, the color point of the emitted green light is shifted to higher x-values, i.e. into the yellow region. By contrast, the color point is shifted towards the green spectral region with a blue color filter layer in the region of the green phosphors. The total range of colors that can be displayed can be increased thereby. In addition, the color of the color picture screen when not in use, the so-called body color, can be varied over a wider range.
It is preferred that the thickness of the red color filter layer in the region of the green phosphors is smaller than the thickness of the red color filter layer in the region of the red phosphors.
It is also preferred that the thickness of the blue color filter layer in the region of the green phosphors is smaller than the thickness of the blue color filter layer in the region of the blue phosphors.
The LCP value is particularly strongly improved in these two embodiments.
It is furthermore preferred that the blue color filter layer comprises a pigment chosen from the group of CoOxe2x80x94Al2O3 and ultramarine pigments.
It is furthermore preferred that the red color filter layer comprises a pigment chosen from the group of Fe2O3, TaON, and CdSxe2x80x94CdSe.
All these pigments are transparent in portions of the emission range of the green phosphor.
It is particularly highly preferred that the pigments have an average particle diameter smaller than 200 nm.
Pigments with a particle diameter smaller than 200 nm show no undesirable scattering of the visible light.
It may be advantageous that a black matrix is provided on the picture screen glass.
A black matrix improves the contrast of the picture displayed on the picture screen in that external incident visible light is absorbed.